Abstract
The synthesis–composition–structure relationship in the Mg–Ca–Al system is studied using combinatorial magnetron sputtering. With increasing deposition temperature, a drastic decrease in Mg concentration is obtained. This behavior can be understood based on density functional theory calculations yielding a desorption energy of 1.9 eV/atom for Mg from a hexagonal Mg nanocluster which is far below the desorption energy of Mg from a Mg2Ca nanocluster (3.4 eV/atom) implying desorption of excess Mg during thin film growth at elevated temperatures. Correlative structural and chemical analysis of binary Mg–Ca thin films suggests the formation of hexagonal Mg2Ca (C14 Laves phase) in a wide Mg/Ca range from 1.7 to 2.2, expanding the to date reported stoichiometry range. Pronounced thermally-induced desorption of Mg is utilized to synthesize stoichiometric (Mg1−x,Alx)2Ca thin films by additional co-sputtering of elemental Al, exhibiting a higher desorption energy (6.7 eV/atom) compared to Mg (3.4 eV/atom) from Mg2Ca, which governs its preferred incorporation during synthesis. X-ray diffraction investigations along the chemical gradient suggest the formation of intermetallic C14 (Mg1–x,Alx)2Ca with a critical aluminum concentration of up to 23 at.%. The introduced synthesis strategy, based on the thermally-induced desorption of weakly bonded species, and the preferential incorporation of strongly bonded species, may also be useful for solubility studies of other phases within this ternary system as well as for other intermetallics with weakly bonded alloying constituents.
Highlights
Hexagonal close-packed magnesium (Mg) combines promising properties for structural applications considering that it exhibits one of the lowest densities among metals (1.7 g/cm3 ) as well as a high specific strength, high abundance, and good castability [1,2]
The results suggest off-stoichiometric formation of intermetallic Mg Ca in the given composition range, extending the off-stoichiometric formation of intermetallic Mg22Ca in the given composition range, extending the range reported by Suzuki et al, who found a solubility of excess Mg in Mg Ca of 4.5 at.% using bulk range reported by Suzuki et al, who found a solubility of excess Mg in Mg22Ca of 4.5 at.% using bulk samples [13]
EV/atom for Mg from a hexagonal Mg nanocluster which is significantly lower than the desorption energy of Mg from a Mg2 Ca nanocluster (3.4 eV/atom), implying temperature-induced desorption of excess Mg during thin film growth at elevated temperatures
Summary
Hexagonal close-packed magnesium (Mg) combines promising properties for structural applications considering that it exhibits one of the lowest densities among metals (1.7 g/cm3 ) as well as a high specific strength, high abundance, and good castability [1,2]. Most Mg–Al alloys undergo a rapid degradation of creep resistance at elevated temperatures restricting their implementation in industry to non-critical parts [1,4,5]. The addition of calcium (Ca) can enhance the creep resistance, which is commonly reasoned by the precipitation of intermetallic (Mg,Al) Ca Laves phases [4,5,6,7,8]. Within the ternary Mg–Al–Ca system, the cubic (C15) Al2 Ca and the hexagonal (C14) Mg2 Ca phases are reported to be the thermodynamically stable binary Laves phases with an extensive solubility of the third constituent, Mg in Al2 Ca and Al in Mg2 Ca [9]. Recent experiments and first-principle calculations suggest the existence of a dihexagonal C36 (Mg,Al) Ca phase [9,10,11,12]
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